1. Field of the Invention
The invention relates in general to a broadband antenna, and more particularly to a broadband antenna which can enlarge antenna bandwidth by a multi-coupling feed-in mechanism.
2. Description of the Related Art
For example, a radio frequency identification (RFID) system transmits identification information by radio waves such that users can obtain the required information by a wireless way. The RFID system is composed of several RFID tags and a reader. Each RFID tag stores the corresponding identification information, such as product names, sources of goods, or purchase dates for product management.
The RFID tag performs data transmission with the reader by using a tag antenna and a tag chip, and whether the RFID tag can efficiently receive energy outputted from the reader determines a read distance between the reader and tag. Therefore, it is a main point in tag antenna design whether the tag antenna and tag chip can have impedance match in order to efficiently receive the energy outputted from the reader. However, owing that the tag chip usually has high capacitive reactance, in order to have conjugate match with the tag chip, the tag antenna is also designed to have high inductive reactance. As a result, an issue of too narrow bandwidth is occurred on the tag antenna. Therefore, a broadband antenna is very important in application of the RFID system.
Besides, the RFID tag chip has impedance variation due to semiconductor process variation, and also varies its impedance as attached to objects of different material, and all the impedance variation needs to be compensated in design of a tag antenna.